It would appear that in the future battery systems will be used increasingly both in stationary applications and in vehicles such as hybrid and electric vehicles. In order to be able to meet requirements set for a respective application in respect of voltage and available power, a high number of battery cells are connected in series. Since the current provided by such a battery needs to flow through all of the battery cells and a battery cell can only conduct a limited current, additional battery cells are often connected in parallel in order to increase the maximum current. This can be provided either by providing a plurality of cell coils within a battery cell housing or by external interconnection of battery cells. However, it is problematic that compensation currents between the battery cells connected in parallel may arise owing to cell capacitances and voltages which are not exactly identical.
The basic circuit diagram of a conventional electric drive unit, as is used, for example, in electric and hybrid vehicles or else in stationary applications such as in the rotor blade adjustment of wind energy installations, is illustrated in FIG. 1. A battery 10 is connected to a DC voltage intermediate circuit, which is buffered by an intermediate circuit capacitor 11. A pulse-operated inverter 12, which provides sinusoidal voltages which are phase-shifted with respect to one another for the operation of an electric drive motor 13 via in each case two switchable semiconductor valves and two diodes at three outputs, is connected to the DC voltage intermediate circuit. The capacitance of the intermediate circuit capacitor 11 needs to be high enough for the voltage in the DC voltage intermediate circuit to be stabilized for a period of time in which one of the switchable semiconductor valves is switched on. In a practical application such as an electric vehicle, a high capacitance in the mF range results.
FIG. 2 shows the battery 10 shown in FIG. 1 in a more detailed block circuit diagram. A large number of battery cells are connected in series and optionally additionally in parallel in order to achieve a high output voltage and battery capacity desired for a respective application. A charging and isolating device is connected between the positive pole of the battery cells and a positive battery terminal 14. Optionally, in addition an isolating device 17 can be connected between the negative pole of the battery cells and a negative battery terminal 15. The isolating and charging device 16 and the isolating device 17 each comprise a contactor 18 and 19, respectively, which are provided for isolating the battery cells from the battery terminals 14, 15 in order to switch said battery terminals to be voltage-free. Owing to the high DC voltage of the series-connected battery cells, there is otherwise a considerable potential risk for maintenance personnel or the like. In addition, a charging contactor 20 with a charging resistor 21 connected in series with the charging contactor 20 is provided in the charging and isolating device 16.
The charging resistor 21 limits a charging current for the intermediate circuit capacitor 11 if the battery is connected to the DC voltage intermediate circuit. For this purpose, first the contactor 18 is left open and only the charging contactor 20 is closed. If the voltage at the positive battery terminal 14 reaches the voltage of the battery cells, the contactor 18 can be closed and possibly the charging contactor 20 can be opened.
The charging contactor 20 and the charging resistor 21 represent significant extra complexity in applications which have a power in the region of a few 10 kW, with this extra complexity being required only for the charging operation of the DC voltage intermediate circuit which lasts a few hundred milliseconds. Said components are not only expensive but are also large and heavy, which is particularly disruptive for the use in mobile applications such as electric motor vehicles.